1. Technical Field
This disclosure relates to a connection apparatus circuit, and more particularly to a connection apparatus circuit provided with protection concept under a high-voltage surge.
2. Description of the Related Art
A connection apparatus is used to be a component adapted to connecting to other devices and is easy to be affected by the surrounding circumstance. Hence, it is important that the connection apparatus or a circuit thereof usually needs to pass any relative identification standards regardless of the physical or electrical concerns, so as to meet the durability requirement and reduce the erroneous rate. For example, a so called DWV test (Dielectric withstanding voltage test) is performed to apply a test voltage higher than the normal working voltage to an electrical terminal of a device under test for the duration in compliance with the standards, and the device passes the test if an electric breakdown never occurs. Moreover, a plurality of withstanding voltage tests, such as an electrostatic discharge test and so on, are performed to examine whether the device is tolerant of different kinds of high-voltage.
The objective of the withstanding voltage tests is to ensure that the electrical terminals outside the device and the circuits inside the device are well electrically insulated. Generally, a high-voltage surge may occur when suffering lightning stroke, electrostatic discharge caused by human or malfunction of a remote apparatus, which may take place in use of the device.
FIG. 1 is a schematic diagram of a connection apparatus circuit 100 of the prior art which is adapted to an Ethernet system or a POTS (Plain Old Telephone Service) system. The connection apparatus circuit 100 comprises an isolation transformer 110, an impedance matching network 120, a high-voltage capacitor 130 and a ground terminal 140, wherein the isolation transformer 110 has a first coil 111 and a second coil 112, and the first coil 111 has a first terminal 113, a second terminal 114 and a center tap 115. The impedance matching network 120 and the high-voltage capacitor 130 are applicable to any matching networks for suppressing common-mode EMI (Electromagnetic Interference). That is, as a common-mode EMI signal exists between the first terminal 113 and the second terminal 114, the impedance matching network 120 and the high-voltage capacitor 130 are integrated to suppress the signal, so as to reduce impacts on the circuit. Besides, the ground terminal 140 is usually coupled to outside casings of the device and connected to the ground point via power lines.
As the FIG. 1 shown, the first terminal 113 and the second terminal 114 are used to be input/output terminals when connecting the connection apparatus to any external devices or circuits, and the signal thereof is a fully-differential AC signal. The second coil 112 is connected to inside circuit of the device. Therefore, the first coil 111 and the second coil 112 not only provide an effective insulation protection on the DC signal, but also transfer the AC signal between the inside circuit and the outside circuit, such that configuration of the connection apparatus circuit is widely used.
As the FIG. 1 shown, when a high-voltage surge occurs between the first terminal 113 and the ground terminal 140, such as suffering lightning stroke or electrostatic discharges caused by human, the high-voltage will conduct a current via the path of minimum impedance of the circuit, as the current path 150 shown, so as to release electrical charges induced by the high-voltage. However, the impedance matching network 120 is not designed to sustain the high-voltage, thus the impedance matching network 120 may suffer damages under several high-voltage surges to form an open circuit. At the same time, if one more high-voltage surge occurs, the current path generated by the high-voltage may change from the path 150 to path 160, which leads to the high-voltage being coupled inside the circuit to damage the circuit. As a result, the durability is poor and the running cost becomes higher.
FIG. 2 is a schematic diagram of another connection apparatus circuit 200 of the prior art. The connection apparatus circuit 200 is provided with two voltage surge suppression units 210, 220 (Transient Voltage Suppressor, TVS), which are connected between the first terminal 113, the second terminal 114 and the ground terminal 140 respectively. Besides, a parasitic resistance 240 is located between the ground point 230 and the ground terminal 140. The voltage surge suppression units 210, 220 are usually constituted of p and n type semiconductors. When a reverse bias voltage thereof is lower than a default value, such as a breakdown voltage, so as to establish equivalent high impedance, and when the reverse bias voltage is higher than the breakdown voltage, so as to establish equivalent low impedance and generate a path for conducting the current.
As the FIG. 2 shown, when a high-voltage occurs between the first terminal 113 and the ground point 230, the high-voltage is higher than the breakdown voltage of the voltage surge suppression unit 210, so as to generate the path 250 for conducting the current. When the high-voltage is caused by the electrostatic, the voltage surge suppression units 210, 220 well perform electrostatic discharges due to the response time not too long. But if the high-voltage is sustained for the duration, such as the first terminal 113 or the second terminal 114 erroneous connected to a 220-volts AC, the path 250 is generated and an AC high-voltage occurs on the ground terminal 140, so as to lead the users to expose to dangers of the electric shock when contacting the casing.